The invention relates to a lithographic method for producing an exposure pattern on a substrate comprising a layer of resist material sensitive to exposure to an energetic radiation, wherein in a pattern transfer system (e.g., a lithographic imaging system) by means of a beam of said energetic radiation a mask having a structure pattern, namely, a set of transparent structures to form a structured beam, is illuminated and the structure pattern is imaged onto the substrate by means of the structured beam, the substrate being positioned after the stencil mask as seen in the optical path of the beam, producing a pattern image, namely, a spatial distribution of irradiation over the substrate, the spatial distribution having a finite pattern transfer blur as determined by the pattern transfer system, and the pattern image is shifted laterally with respect to the substrate between a plurality of predetermined shift positions and with each shift position the substrate is irradiated for a predetermined time, the width of lateral displacements between two neighboring shift positions being smaller than the minimum feature size of the exposure pattern to be formed.
In manufacturing semiconductor devices, one important step for structuring the semiconductor substrates is lithography. The substrate, for instance a silicon wafer, is coated with a thin layer of photosensitive material, called photo-resist. By means of a lithographic imaging system, a pattern is imaged onto the photo-resist, and the subsequent development step removes from the substrate either the exposed or the unexposed portion of the photo-resist. Then, the substrate is subjected to a process step such a etching, deposition, oxidation, doping or the like, the photo-resist pattern on the substrate covering those portions of the surface that shall remain unprocessed. The photo-resist is stripped, leaving the substrate with the new structure. By repeating this sequence, multiple structure layers can be introduced to form the semiconductor micro-circuits.
Lithographic projection methods and lithographic devices using electron beams are discussed, for instance, in H. Koops, xe2x80x9cELECTRON BEAM PROJECTION TECHNIQUESxe2x80x9d 235-255, FINE LINE LITHOGRAPHY (Roger Newman ed. 1980). Electrons, and in particular ions, have the advantage of very low particle wavelengthsxe2x80x94far below the nanometer rangexe2x80x94which allows for very good imaging properties, as, for example, discussed in Gerhard Gross et al., xe2x80x9cIon projection lithography: Status of the MEDEA project and United States/European cooperation,xe2x80x9d J. VAC. SCI. TECHNOL., B16(6), pp. 3150-3153, Nov./Dec. 1998.
In projection lithography, the pattern to be imaged onto the photoresist-covered substrate is produced by using a mask or recticle having the desired pattern. For particle projection systems, stencil masks are used in which the patterns to be projected are formed as apertures of appropriate shape in a thin membrane, i.e., a few micrometers thick. The mask pattern is built up from a number of apertures in a thin membrane through which the particle beam is transmitted to expose the resist-coated wafer in those areas required for device fabrication.
Stencil masks may also be used in lithography systems based on photons, like EUV (Extreme UV) or X-ray lithography, e.g. in the EUV lithography system transmission mask geometry as proposed by H. Lxc3x6schner et al. in the U.S. application Ser. No. 09/316,834.
A further application of the present invention is for the stencil masks used in vapor deposition systems, e.g. in nanosieves as put forward in J. Brugger et al., xe2x80x9cRESISTLES 100-nm PATTERN FORMATION USING NANOSIEVES AS SHADOW MASKSxe2x80x9d, INTERNATIONAL CONFERENCE ON MICRO- AND NANO-ENGINEERING ABSTRACT BOOK, Rome, Italy, 193-194, Sep. 21-23, 1999. In this case, the particles are essentially neutral atoms or molecules. In this case, the particles are essentially neutral atoms or molecules.
As also discussed by H. Koops, op. cit., pp. 245-248, with self-supporting stencil masks a problem arises for configurations which require a ring-shaped exposure region on the wafer; the central area of the ring-shaped region is completely surrounded by the aperture (so-called doughnut problem) and thus xe2x80x98cut outxe2x80x99. Problems also arise with simply connected patterns like free-standing bars or leafs. Therefore, additional means need to be taken to stabilize the central area in the proper position of the mask. Also for large or very long aperture areas in the mask, which effectively separate the mask foil into distinct parts, a stability problem arises; moreover, these structures are difficult to prepare.
Therefore, a xe2x80x9ccrossed grating solutionxe2x80x9d was proposed for electron projecting systems, where a supporting grid of the finest grid constant which can be generated is overlaid on the desired pattern for device fabrication. If the supporting grid constant is about one tenth of the finest line-width in the mask, the supporting grid will vanish in the exposed and developed area, due to the proximity effect and the limited resolution of the projecting optical system. Due to the fine dimensions of the supporting grid, however, its production is too difficult to be practically implemented.
Another approach, presented, for example, in U Behringer and H. Engelke, xe2x80x9cIntelligent Design Splitting in the Stencil Mask Technology Used for Electron- and Ion-Beam Lithography,xe2x80x9d J. VAC. SCI. TECHNOL. B11(6), pp. 2400-2403, Nov./Dec. 1993, splits the device pattern into complementary mask fields situated on at least two masks. Thus, the pattern on each complementary mask is more stable; however, now a set of masks has to be handled with the lithography setup. Also, the production expenses of the stencil masks are multiplied accordingly.
The so-called xe2x80x9cmultibeamxe2x80x9d solution, also described by H. Koops, op. cit., subdivides the device pattern into squares of equal area by a software routine; for each square of the device pattern, an aperture is provided in the aperture pattern which, though, only covers a fraction, e.g., a quarter, of the device pattern square. This is illustrated in FIG. 1 with a device pattern DP having the shape of a rectangular line. The structure of the pattern is subdivided into quadratic areas PQ which each correspond to a quarter of the smallest elements in the device pattern desired. With a line as in FIG. 1, a square has a side length ps equal to half the line width dw; typical values of these dimensions are, for example, ps=100 nm and dw=200 nm. The substrate is multiply exposed with this aperture pattern, in the example of FIG. 1 four times where one square PQ is shifted laterally, as indicated by the arrow-line is, to the four quadrants of a square of doubled side length, pd=2 ps, and the total pattern is constructed by subsequent exposures of the wafer; for each shift position, the same duration of irradiation is used. Within this disclosure, a xe2x80x98lateralxe2x80x99 movement means a movement along thexe2x80x94usually flatxe2x80x94surface of the substrate or mask, as the case my be. In the pattern thus produced, the adjacent images PQxe2x80x2 of the aperture squares PQ lie side by side, as illustrated in FIG. 1a. It should be noted that the dimensions of the imaged pattern depend on whether the imaging optics is a 1:1 optics or has a demagnification, e.g. by a factor of 4. By virtue of the xe2x80x9cmultibeamxe2x80x9d solution, a plurality of small apertures is realized instead of a large opening in the foil, and the remaining foil forms stable struts between the apertures which improves the mechanical stability of the mask and eases preparation of the masks.
An electron microprojector setup and mask geometries exploiting the xe2x80x9cmultibeamxe2x80x9d method are also described in J. Frosien et al., xe2x80x9cApplication of the electron microprojector in the field of microlithography,xe2x80x9d PROCEEDINGS OF THE MICROCIRCUIT ENGINEERING ""79, Rhienisch-Westfalische Technische Hochschule, Aachen, Germany, Sep. 25-27, 1979.
However, the xe2x80x9cmultibeamxe2x80x9d solution has been rejected for the use in semiconductor lines since it appeared that only straight lines parallel to one of the directions of the lateral shift and, moreover, only structures having dimensions which are integer multiples of the distance pd between neighboring aperture openings could be obtained. Moreover, it is impossible to compose from xe2x80x98orthonormalxe2x80x99 squares as defined by the lateral shift displacements, such as those PQxe2x80x2 of FIG. 1a, a sufficiently smooth edge running along an inclined linexe2x80x94a problem very similar to the problem well known from digitalization of images into raster graphics.
It is an aim of the present invention to overcome these restrictions of the xe2x80x9cmultibeamxe2x80x9d solution and to offer a way for a better design flexibility. In particular the invention is aimed at using a single mask for producing structure which are, within prescribed limits of geometrical accuracy, slanted with respect to the direction of the lateral movement and structures of arbitrary dimensions with a particle-beam lithography projection setup.
This aim is met by a lithographic method as stated in the beginning wherein, according to the invention, the dimension and/or direction of at least one structure of the structure pattern is incongruent with respect to the lateral shift displacements between shift positions, and the pattern transfer blur is not smaller than the width of lateral displacements between neighboring shift positions, the exposures with respect to the plurality of shift positions superposing into a spatial distribution of exposure dose on the substrate, said distribution exceeding the specific minimum exposure dose of said resist material within only one or more regions of the substrate, said region(s) forming the exposure pattern.
This solution makes it possible to produce device patterns which cover large areas or are enclosing free-standing areas without endangering the structural stability of the mask. The average void ratio of the stencil mask according to the invention is small, typically smaller than 12.5%, thus the local anisotropy of the foil properties can be kept small. Thus the invention allows of a xe2x80x98self-complementary maskxe2x80x99, by means of which the desired exposure pattern is composed from a set of images of the very same mask pattern.
The pattern transfer blur is chosen according to the desired xe2x80x98smear-outxe2x80x99 with respect to the incongruent pattern structuring of the mask actually used. In the context of this disclosure, the pattern transfer blur is defined as the width at half the maximum of the irradiation spread function, where the latter corresponds to the spatial distribution of irradiation on the substrate produced by a point-like aperture (or point-like reflective spot, as the case may be). For the sake of brevity, in the following the simple reference to the xe2x80x98blurxe2x80x99 will refer to the pattern transfer blur unless indicated otherwise. If, e.g., the irradiation spread function has a Gaussian shape, the blur is 2.36 times the standard deviation "sgr" of the Gaussian distribution.
In a preferred embodiment of the invention, the pattern transfer blur is advantageously in the range of 1.4 to 1.8 times the width of lateral displacements between neighboring shift positions.
One method for performing the lateral displacement of the image pattern over the substrate employs adjustment of the optical properties of the pattern transfer system which usually can be adjusted with a high accuracy, in particular in the case of a particle optical system. Thus the pattern image is laterally shifted with respect to the substrate by adjustment of the optical properties of the pattern transfer system. This way of lateral shift allows a high precision of the displacement of the image pattern while the mask and the substrate can rest still during the exposure procedure.
In a preferred embodiment the energetic radiation comprises electrically charged particles, and the pattern transfer system is a particle optical imaging system. This makes it possible to realize a very low numerical aperture which leads to a high depth of focus at the image plane, which is especially advantageous if the substrate is non planar, e.g. if it is structured vertically as well. Preferably, the radiation comprises ions, such as hydrogen or helium ions, and the pattern transfer system is an ion optical imaging system. It is noteworthy that the particles used for the lithography beam can be any electrically charged species, in particular ions which are primarily used by the applicants. In comparison to electrons, ions offer even more advantageous values of optical parameters, for instance, with 10 keV protons, the wavelength is approximately 0.05 pm; a typical value of numerical aperture is 10xe2x88x925, and depth of focus 500 xcexcm. With a particle lithography system, the lateral shift of the pattern image can advantageously be performed by an electrostatic multiple means of the particle optical imaging system, as used, for instance, in the masked-beam system disclosed by Stengl et al. in the U.S. Pat. No. 5,742,062.
Another variable method for performing the lateral shift of the image pattern with respect to the substrate is by laterally shifting the substrate and/or the mask. The controlled movement of the substrate, the mask, or a combination of both allows a direct control of the displacement of the pattern image on the substrate.
Further, it is advantageous if the total area occupied by the structures in the maskxe2x80x94within the totality of the structure pattern formed in the mask or one or more parts of the structure patternxe2x80x94is smaller than the area corresponding to the exposure pattern to be formed on the substrate divided by the number of lateral shifts. By this measure, the stability of the mask is further increased; in charged particle lithography, also the total current required to form the exposure pattern on the substrate is reduced, thus increasing the throughput of the system.
The device pattern elements to be produced usually have rectangular or polygonal shapes. Correspondingly, the pattern structures in the mask can be of rectangular shape. Rectangular pattern structure are easy to process with the software for designing the mask pattern. Alternatively, the pattern structures in the mask may be of rounded shape. Circular or other rounded structures are earlier to produce with known methods of structuring. Furthermore, in particular in the case of circular structure elements, the orientation of the elements needs not to be taken into account, it should be noted in this context that, as discussed more in detail below, the details of the pattern structure shapes are of minor importance as compared to the overall layout of the structures.
The lithographic system in which the invention is realized can be a demagnifying particle projection system, such as the 4xc3x97demagnifying system as described in detail by Stengl et al. in the U.S. Pat. No. 4,985,634. FIG. 10a shows in graphical symbol form an example of a demagnifying particle system 100 for imaging an exposure pattern from a mask 103 onto a substrate WF. An ion beam source S projects the beam 101 through various components of the demagnifying particle system 100, optionally including an electrostatic multipole means 102, to produce the beam 101 shown in FIG. 10a. The system 100 may also contain various other components (not shown) known to those skilled in the art. Since the structure pattern elements of the mask for a demagnifying system have greater dimensions, production of the mask is facilitated. Moreover, the flow density of the particle beam at the mask can be reduced accordingly. Another lithography system suitable for the invention may be a 1:1 ion shadow projection system, for instance, the above-mentioned masked-beam system of Stengl et al., U.S. Pat. No. 5,742,062. FIG. 10b shows in graphical symbol form an example of a 1:1 ion shadow projection system 200 for producing an exposure pattern defined in a mask 203 on a substrate WF. A beam source S projects a beam 201 through various components of the system 200, which as discussed above, may optionally contain an electrostatic multipole means 202, along with various other components (not shown) known to those skilled in the art.